CL/VK: Add helper to select LWS In the case where user passes NULL for LWS, the runtime has the responsibility to select an appropriate LWS for user. Bug: angleproject:8631 Change-Id: I89048c57880a5a4f18d4ef46340b73338cee933e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5421573 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Austin Annestrand <a.annestrand@samsung.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>